This invention relates to a DC three-wire circuit for a vehicle, which includes a starter motor for starting an internal combustion engine installed on the vehicle; in which the batteries are charged by charging generators driven by the internal combustion engine; and in which the rectifiers of the charging generators and the starter motor are protected from damage while the batteries are maintained charged.
In a proposed DC three-wire circuit of this type, the protection of the rectifiers in the charging generators from damage which may occur because of a defective contact of the battery switch; the protection of the starter motor from damage by automatically disconnecting it from the battery when the speed of the engine reaches a predetermined value, and the prevention of the discharge of the batteries which may occur when the key switches remain turned on after the engine stops, are carried out by electromagnetic relays, namely, a battery relay, a starter safety relay and a field relay, respectively.
However, this system is disadvantageous in that the external connection is intricate and a relatively large space is required for installation.